Who among us hasn't quickly reclaimed a floor cookie when no one was looking? If nobody's seen: it didn't happen and it's not dirty. Well, not quite.

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A new study out of Manchester Metropolitan University has come up with a more nuanced iteration of the fallen food rule. Because, if you're going to do it, you might as well know with which foods you'll be safe(er).

The food sciences researches looked at five test foods—bread with jam, cooked pasta, a slice of ham, a plain cookie, and dried fruit—which were left of on the ground for intervals of three, five and 10-seconds. These commonly eaten foods were selected because of their variance in water-activity level—water being a key factor in bacteria growth.

What was found

• Foods with a high salt or sugar content, like many processed foods, were less likely to pick up harmful bacteria in so short a period of time. The ham (high salt) and sugary bread and jam were both considered safe, with little sign of bacterial growth, when retrieved after three seconds on the ground. The jam's high sugar content made the slathered slice inhospitable for bacterial growth.
• The cooked pasta and dried fruit, however, both tested positive for klebsiella, a bacteria known to cause a range of diseases and infections, as well as soft-tissue conditions. (Surprising. You'd think filth and germs would be more likely to stick to wettish ham, as compared to dried fruit. But it's the salt and nitrates in the ham, a preserved meat, that protect it from growth of most bacteria.)
• The dry cookie was also relatively well off—after three, five and ten seconds—because of its low water activity level and "adhesion ability".